One of the most promising applications in the field of communications is the transmission of encoded or compressed video information across mediums such as telephone networks, cable television networks, and the Internet. The video information is typically encoded at one location, transferred over a network to a destination, and decoded at the destination. A video encoder may be used to, among other things, receive video signals and other data, convert these to digitized form (if the data is not already in digitized form), and compress the data using an encoding algorithm. Examples of video encoding algorithms are the MPEG-1, MPEG-2, and MPEG-4 digital video-coding standards of the Motion Picture Expert Group and the H.261 and H.264 standards from the ITU-T (International Telecommunications Union-Telecommunications sector). The video information may be first obtained from a video camera that generates video signals which contain a series of video frames that each depict images. In addition, the video-coded information may also include other components such as, for example, animations, text, and/or audio information. A video decoder may be used at the destination to decode the received video-coded frames into a format that may be displayed by a video monitor or other device.
The networks over which video-coded information is transmitted generally have limited bandwidth. The amount of bandwidth available at any given time depends upon factors such as the amount of traffic and the operability of network components. The amount of available bandwidth is a major parameter for the quality of the video presented by the video decoder, with an increase in bandwidth increasing the quality of the video. The unavailability of bandwidth may cause video-coded frames to be lost or delayed while being sent across the network due to network congestion or buffer overflow. Such losses and delays may cause severe degradation in perceived quality of the video displayed by the video decoder and may result in error propagation, especially where the video is sent in real-time or streamed or where the video contains objects in motion.
One way to combat the impact of limited bandwidth is to use a “layered” video-coding scheme. Layered video-coding schemes divide a stream of video-coded frames into layers. In the standard layered video-coding scheme, the video-coded information signal is divided into a base layer and an enhancement layer. The enhancement layer typically provides refinement information that is added to the base layer to provide higher quality. For example, the base layer may typically be decoded and displayed without any data from the enhancement layer, although this would probably result in reduced picture quality. When network congestion occurs, packets may be dropped from the enhancement layer, but valid video at a slightly lower quality may still be produced by decoding packets from the base layer.
Traffic that is sent over a network is often classified by a priority level. In many networks, the priority level will be used if the network needs to drop some of the packets being transported. For example, if network congestion occurs and the network has to drop packets, the network will typically drop the lower level packets. A video encoder may take advantage of these classifications by designating some video-coded frames as high priority and the rest as low priority. For example, every fifth frame may be sent as high priority, with the rest being sent as low priority. A typical layered video-coding scheme is to transmit enhancement layer traffic over the network with a lower priority than the base layer. Another scheme, which is referred to as using multiple threads, codes the base layer frames independently, using only previous base layer frames for motion compensation, and codes the enhancement layer frames normally, using both base and enhancement layer frames for motion compensation.
A problem may occur when using a layered video-coding scheme if there are simultaneous attempts to transmit high priority base layer frames. If this occurs, the network may become overloaded with high priority data. In this case, either base layer frames may be lost, causing drastic quality reduction, or the network may be forced to buffer some of the base layer frames, causing excessive delays.